An electrophotographic light-sensitive material may have various structures depending upon the characteristics required or an electrophotographic process to be employed.
An electrophotographic system in which the light-sensitive material comprises a support having thereon at least one photoconductive layer and, if necessary, an insulating layer on the surface thereof is widely employed. The electrophotographic light-sensitive material comprising a support and at least one photoconductive layer formed thereon is used for the image formation by an ordinary electrophotographic process including electrostatic charging, imagewise exposure, development, and, if desired, transfer.
Furthermore, a process using an electrophotographic light-sensitive material as an offset master plate precursor for direct plate making is widely practiced. In particular, a direct electrophotographic lithographic plate has recently become important as a system for printing in the order of from several hundreds to several thousands of prints having a high image quality.
Binders which are used for forming the photoconductive layer of an electrophotographic light-sensitive material are required to be excellent in the film-forming properties by themselves and to possess the capability of dispersing photoconductive powder therein. Also, the photoconductive layer formed using the binder is required to have satisfactory adhesion to a base material or support. Further, the photoconductive layer formed by using the binder is required to have various excellent electrostatic characteristics such as high charging capacity, small dark decay, large light decay, and less fatigue before light-exposure and also have an excellent image forming properties, and the photoconductive layer stably maintains these electrostatic properties to change of humidity at the time of image formation.
Further, extensive studies have been made for lithographic printing plate precursors using an electrophotographic light-sensitive material, and for such a purpose, binder resins for a photoconductive layer which satisfy both the electrostatic characteristics as an electrophotographic light-sensitive material and printing properties as a printing plate precursor are required.
However, conventional binder resins used for electrophotographic light-sensitive materials have various problems particularly in electrostatic characteristics such as charging property, dark charge retention, light sensitivity, etc., and smoothness of the photoconductive layer.
In order to overcome the above problems, JP-A-63-2173 and JP-A-1-70761 (the term "JP-A" as used herein means an "unexamined Japanese patent application") disclose improvements in the smoothness of the photoconductive layer and electrostatic characteristics by using, as a binder resin, a resin having a low molecular weight containing from 0.05 to 10% by weight of a copolymer component containing an acidic group in side chains of the polymer or a resin having a low molecular weight (i.e., a weight average molecular weight (Mw) of from 1.times.10.sup.3 to 1.times.10.sup.4) having an acidic group bonded at the terminal of the polymer main chain thereby obtaining an image having no background stains. Also, JP-A-1-100554 and JP-A-1-214865 disclose a technique using, as a binder resin, a resin containing a polymer component containing an acidic group in side chains of the copolymer or at the terminal of the polymer main chain, and containing a polymer component having a heat- and/or photo-curable functional groups; JP-A-1-102573 and JP-A-2-874 disclose a technique using a resin containing an acidic group in side chains of the copolymer or at the terminal of the polymer main chain, and a crosslinking agent in combination; JP-A-64-564, JP-A-63-220149, JP-A-63-220148, JP-A-1-280761, JP-A-1-116643 and JP-A-1-169455 disclose a technique using a resin having a low molecular weight (a weight average molecular weight of from 1.times.10.sup.3 to 1.times.10.sup.4) and a resin having a high molecular weight (a weight average molecular weight of 1.times.10.sup.4 or more) in combination; and JP-A-2 -34859 discloses a technique using the above low molecular weight resin and a heat- and/or photo-curable resin in combination. The above prior art references disclose that, according to the proposed technique, the film strength of the photoconductive layer can be increased sufficiently and also the mechanical strength of the photosensitive material can be increased without adversely affecting the above-described electrostatic characteristics by using a resin containing an acidic group in side chains or at the terminal of the polymer main chain.
However, it has been found that, even in the case of using these resins, it is yet insufficient to keep the stable performance in the case of greatly changing the environmental conditions from high-temperature and high-humidity to low-temperature and low-humidity. In particular, in a scanning exposure system using a semiconductor laser beam, the exposure time becomes longer and also there is a restriction on the exposure intensity as compared to a conventional overall simultaneous exposure system using a visible light, and hence a higher performance has been required for the electrostatic characteristics, in particular, the dark charge retention characteristics and photosensitivity.
Further, when the scanning exposure system using a semiconductor laser beam is applied to hitherto known light-sensitive materials for electrophotographic lithographic printing master plates, various problems may occur in that the difference between E.sub.1/2 and E.sub.1/10 is particularly large and the contrast of the reproduced image is decreased. Thus, it is difficult to reduce the remaining potential after exposure, which results in severe fog formation in duplicated images, and when employed as offset masters, edge marks of originals pasted up appear on the prints, in addition to the insufficient electrostatic characteristics described above.